1. Field of Invention
The present invention relates to an apparatus and method for stimulating a subterranean formation(s), and more particularly, to such an apparatus and method wherein a propellant is employed to stimulate the subterranean formation(s) and/or to enhance the effectiveness of perforations which provide communication between a well and the formation(s).
2. Description of Related Art
Individual lengths of relatively large diameter metal tubulars are secured together to form a casing string which is positioned within a subterranean well bore to increase the integrity of the well bore and provide a path for producing fluids to the surface. Conventionally, the casing is cemented to the well bore face and subsequently perforated by detonating shaped explosive charges. These perforations extend through the casing and cement a short distance into the formation. In certain instances, it is desirable to conduct such perforating operations with the pressure in the well being overbalanced with respect to the formation pressure. Under certain overbalanced conditions, the well pressure exceeds the pressure at which the formation will fracture, and therefor, hydraulic fracturing occurs in the vicinity of the perforations. As an example, the perforations may penetrate several inches into the formation, and the fracture network may extend several feet into the formation. Thus, an enlarged conduit can be created for fluid flow between the formation and the well, and well productivity may be significantly increased by deliberately inducing fractures at the perforations. Gas generating propellants have been utilized in lieu of hydraulic fracturing techniques as a more cost effective manner to create and propagate fractures in a subterranean formation. In accordance with conventional propellant stimulation techniques, a propellant is ignited to pressurize the perforated subterranean interval either simultaneous with or after the perforating step so as to propagate fractures therein. One propellant stimulation tool consists of a cast cylinder of solid rocket propellant having a central ignition system which consists of a detonator cord positioned within a hole formed in the center of the cylinder of propellant. The hole may be provided with a thin walled aluminum or cardboard tube to assist in insertion of the detonator cord. However, this propellant stimulation tool lacks sufficient mechanical strength to withstand the forces encountered when run into a small diameter well bore, in particular a well bore which is deviated, and/or when the well bore temperature exceeds about 275.degree.F. In an attempt to increase tool integrity during use in larger diameter well bores, a relatively large, heavy carrier, e.g. 31/2" outer diameter and 3/8' thick, is positioned around the cylinder of propellant. However, the weight of this tool, e.g. 200 lbs. for a 20 foot carrier, inhibits the use thereof, especially in deviated well bores. Thus, none of these prior art devices which utilized propellants in stimulation tools have provided completely satisfactory results in well bores of varying diameters or a repeatable and reliable propellant burn in a discrete or controlled pattern. In view of this, a need exists for a propellant stimulation tool which possesses sufficient structural integrity as manufactured to be employed in vertical or deviated well bores of varying diameters and which provides an internally discrete and controlled burn pattern upon ignition of the propellant.
Thus, it is an object of the present invention to provide an apparatus for stimulating a subterranean formation utilizing a propellant in which the apparatus has a high degree of structural integrity.
It is also object of the present invention to provide an apparatus for stimulating a subterranean formation utilizing a solid mass of propellant which results in relatively no debris upon ignition.
It is another object of the present invention to provide an apparatus for stimulating a subterranean formation in which the number and position of the ignition points for propellant which is utilized in the apparatus is controlled thereby achieving a substantially repeatable burn of the propellant.
It is a further object of the present invention to provide an apparatus for stimulating a subterranean formation which can be utilized at relatively high temperatures.